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converts Kretschmann scalars in expr to Riemann tensors.
KretschmannToRiemann[expr, cd]
only converts curvature tensors of the covariant derivative cd.
  • The Kretschmann scalar is the fully contracted square of the Riemann tensor, K = R_(ab)^(cd)R_(cd)^(ab).
  • The result is wrapped in a Scalar head to prevent index collisions.